Determination of oxygen in molten steel

ABSTRACT

This invention is concerned with a process of very accurately and rapidly measuring the oxygen content of molten steel as the steel is being processed. This invention is dependent for its viability upon the use of a solid electrolyte comprising stabilized zirconia and beryllia reacted at a high temperature to form such solid electrolyte. Controlled porosity of the solid electrolyte forms a part of this invention.

nited States Patent Jackson 1 July 4, 1972 [54] DETERMINATION OF OXYGEN IN 3,400,054 9/1968 Ruka et al. ..204/] T MoLTEN STEEL 3,403,090 9/1968 Tajiri et a1.

3,468,780 9/1969 Fischer [72] Paul 3,481,855 12/1969 Kolodney 6t a1 ..204/195 s [73] Assignee: Ford Motor Company, Dearborn, Mich.

Primary Examiner-T. Tung [22] Flled' 1970 Attorney-John R Faulkner and Thomas H. Oster [21] Appl. No.: 97,350

[57] ABSTRACT U68. T invention is concerned a process of very accurately "f 6 -G01n 27/6 and rapidly measuring the oxygen content of molten steel as [58] Fleld of Search "136/86 86 F? 204/ l 195 S the steel is being processed. This invention is dependent for its viability upon the use of a solid electrolyte comprising stabil- [56] References cued ized zirconia and beryllia reacted at a high temperature to UNITED STATES PATENTS form such solid e1ectrolyte. C ontro1 led porosity of the solid electrolyte forms a part of this invention.

3,216,911 11/1965 Kronenberg ..204/195 S 3,350,230 10/1967 Tannenberger ..136/86 F 5 Claims, No Drawings DETERMINATION OF OXYGEN IN MOLTEN STEEL THE STATE OF THE ART NOW The production of a large portion of the sheet steel in this country is basically a process for the oxidation of carbon from a molten bath by exposure to elemental oxygen. Where a superior surface finish and yield is desired the usual product is rimming steel in which the impurities in the solid ingot are entrapped in the central portion of the ingot leaving a skin or surface of substantially pure ferrite.

Those who labor in this field are painfully aware of the difficulty of obtaining molten metal in the teeming ladle with precisely the correct oxygen content, or stated otherwise, the proper equilibrium between carbon and oxygen in the molten metal. The desired oxygen content usually ranges between 0.01 percent and 0.10 percent, depending upon the carbon content and class of steel being produced. Great economies could be effected in the steel industry if the precise oxygen content of the steel could be ascertained essentially instantaneously and economically, so that the necessary corrections could be made.

A detailed exposition of the efforts toward this end is described in US. Pat. No; 3,468,780 issued Sept. 23, 1969 to Fischer and in an article' appearing in Volume 245 of Transactions of the Metallurgical Society of AIME dated July 1969 at pages 1,501 to 1,509. The authors of this paper are Fruehan, Martonik and Turkdogan. The Fruehan et al., paper teaches in the initial paragraph the concept of an electrolytic cell for the measurement of oxygen in steel in which the solid electrolyte is stabilized zirconia contacted on one side by molten steel and on the other side by an oxygen pressure reference material which takes the form of a mixture of fine chromic oxide and chromium metal. The stabilized zirconia electrolyte is in the form of a disc, saiddisc being sealed in one end of a silica tube which contains the Cr/Cr O mixture. This arrangement mitigates the thermal shock problem inherent in the use of the stabilized zirconia tubes taught by Fischer. However, the possibility of electrically short circuiting the cell due to penetration of the zirconia/silica seal by the liquid steel exists, and the maximum temperature at which the cell may be used is limited by the melting temperature of silica.

The efforts of the prior art have all suffered from the obvious difficulties of establishing a precise electrical system capable of immersion into slag and steel at temperatures ranging up to 3,200 F. and still remaining intact both mechanically and electrically. These difiiculties together with a general lack of reproducibility of results at the higher oxygen levels have prevented extensive use of prior art devices.

THE INVENTION The term stabilized zirconia is thought to be sufficiently familiar to ceramicists to require little further elaboration. A complete treatise on the subject of stabilized zirconia will be found in a publication entitled Refractories by F. H. Norton (Fourth Edition). In reference to zirconia as a refractory Norton makes the following statement:

If certain oxides such as MgO and CaO are present, a cubic form is obtained at about 1,700" C. (about 3,090 F.). This isometric form does not transform on cooling and is obtained only in the presence of oxide impurities (other types of impurities might act in the same way.)

In addition to the magnesia and lime mentioned by Norton yttria and ceria have been found to be useful in stabilizing zirconia.

Stabilized zirconia is known to be much more resistant to thermal shock than pure zirconia. However, by comparison with a number of other refractories stabilized zirconia is not considered to have superior thermal shock resistance. This invention is predicated upon the discovery that beryllia can be reacted with either zirconia or stabilized zirconia to yield new classes of refractories difierent from either parent refractory which possess excellent resistance to thermal shock when suddenly exposed to very high temperatures. For the case of beryllia reacted with stabilized zirconia there are also highly unexpected and beneficial results with regard to the electrical properties of the resultant refractory when it is employed as a solid electrolyte. Beneficial results have been obtained when beryllia is reacted with stabilized zirconia so that the resultant product contains between 15 and mol percent beryllia. Lime is the preferred stabilizing agent.

The greatest uniformity in the electrical properties of the resultant product is obtained when the reaction is carried out at very high temperatures; melting together of lime, beryllia and zirconia would be an extreme example. From the economic standpoint, however, it has been found convenient to homogenize beryllia and stabilized zirconia powders and to react the resultant mixture by sintering. It has been found that the sacrifice in uniformity of the electrical properties of the resultant solid electrolyte is small provided that the sintering is carried out at a high enough temperature. Such a sintered solid electrolyte is highly stable to thermal shock and highly conductive to heat to minimize thermoelectric effects resulting from temperature gradients across the electrolyte and to decrease response time of the cell. It also has the unique quality, when used to measure oxygen levels in liquid steel, of possessing a temperature coeflicient for the electromotive force resulting from different oxygen activities on either side of the electrolyte, which is only about one-half the corresponding value found for stabilized zirconia. Since temperatures are difficult to measure with great certainty in liquid steel not only because of problems with high temperature thermocouples but also because of temperature variations within the molten mass, insensitivity of the electrical readout value to temperature variation of the steel is clearly of considerable practical importance.

A further benefit of this invention is that the resultant solid electrolyte because of its thermal shock resistance and ease of fabrication into various shapes can be produced and used directly in the form of closed end tubes. Besides the obvious economic advantages in avoiding the necessity of making seals between fused silica and stabilized zirconia as suggested by Fruehan, et al., and others, the temperature limitation inherent in the use of fused silica in contact with molten steel is avoided. The maximum short time service temperature of fused silica is fixed by its relatively low melting point of about 3,100 F. The melting points of the refractories produced by reacting beryllia with lime stabilized zirconia are all above 3,600 F. Thus the useful temperature range of oxygen measurement in steel may be extended with the instant invention by several hundred degrees.

In addition to these advantages I have found that solid electrolytes which are the reaction product of lime, beryllia and zirconia offer greater reproducibility and a higher level of predictability when measuring the high oxygen levels commonly found in low carbon steels produced in the Basic Oxygen Furnace than has been ascribed to other solid electrolyte devices.

To overall properties of the reacted solid electrolyte refractory are improved if a certain porosity is incorporated in the finished product. This is accomplished by the addition of a material such as powdered graphite or other heat destructible material to the mix prior to sintering. These voids should form a discontinuous phase. The lubricant and bonding agent employed in the pressing operation should be one which will give a sintered product of very high density after compacting at moderate pressures. Naphthenic acid is an example of such a bonding material.

A SPECIFIC EXAMPLE Tubular solid electrolytes were pressed which measured about five-sixteenths of an inch in diameter by 1% inches long. These tubes were hollow and closed at one end. The inside hole diameter was about three-sixteenths of an inch. After the initial burning out of the graphite and naphthenic acid, they were trimmed to insure a unifonn endwall thickness; then refired, in air, at a high temperature.

This process started with the milling together of beryllia, stabilized zirconia, graphite and naphthenic acid in benzene. The grinding balls where alumina. The amount of beryllia added was such that the final product contained about 70 mol percent beryllia. The graphite employed was about 20 percent of the oxides by volume. On the basis of the oxides about three weight percent of naphthenic acid was employed.

The contents of the ball mill were separated from the balls, dried, crushed and pressed into closed end tubes at a pressure of about 1,500 pounds per square inch. These green cylinders were very carefully heated to burn out the graphite and the naphthenic acid. The final temperature reached in this initial firing was 2,200 F. After cooling, these preforms were trimmed to insure a uniform endwall thickness. They were then sintered at a carefully controlled temperature.

In this particular example, the temperature range which was found to be above the minimum temperature for adequate reaction of the oxides but below the temperature at which the thermal shock resistance was curtailed due to elimination of the pores left by the graphite particles was from 1,650 to l,700 C. It is advisable to employ the lowest sintering temperature within this range which will ensure a satisfactory lack of permeability in the finished product. The degree of permeability may be readily observed by wetting one side of a sintered tube with a black dye, sectioning the tube, and observing the penetration of the dye. The sintering temperature adopted is the lowest one at which the tubes still show negligible throughgoing porosity. The required sintering temperature is found to increase somewhat as the percentage of beryllia is lowered.

I claim as my invention:

1. The process of determining the oxygen content of molten steel comprising employing as a solid electrolyte a sintered composition consisting essentially of stabilized zirconia and beryllia, the beryllia constitution from 15 to mol percent of the total, exposing said solid electrolyte on one side to molten steel and on the other side to an oxygen pressure reference material, and measuring the electrical potential developed across said solid electrolyte.

2. The process described in claim 1 in which the zirconia is stabilized by calcium oxide.

3. The process described in claim 1 in which the sintered solid electrolyte contains a substantial portion of voids for the purpose of enhancing the resistance of the solid electrolyte to thermal shock.

4. The process of determining the oxygen content of molten steel comprising employing as a solid electrolyte a sintered composition consisting essentially of beryllia and zirconia stabilized by an oxide of the group consisting of magnesia, yttria and ceria, said beryllia constitution from 15 to 85 mol percent of the total, exposing said solid electrolyte on one side to molten steel and on the other side to an oxygen pressure reference material, and measuring the electrical potential developed across said solid electrolyte.

5. The process described in claim 4 in which the sintered solid electrolyte contains a substantial portion of voids for the purpose of enhancing the resistance of the solid electrolyte to thermal shock. 

2. The process described in claim 1 in which the zirconia is stabilized by calcium oxide.
 3. The process described in claim 1 in which the sintered solid electrolyte contains a substantial portion of voids for the purpose of enhancing the resistance of the solid electrolyte to thermal shock.
 4. The process of determining the oxygen content of molten steel comprising employing as a solid electrolyte a sintered composition consisting essentially of beryllia and zirconia stabilized by an oxide of the group consisting of magnesia, yttria and ceria, said beryllia constitution from 15 to 85 mol percent of the total, exposing said solid electrolyte on one side to molten steel and on the other side to an oxygen pressure reference material, and measuring the electrical potential developed across said solid electrolyte.
 5. The process described in claim 4 in which the sintered solid electrolyte contains a substantial portion of voids for the purpose of enhancing the resistance of the solid electrolyte to thermal shock. 